1. Field of the Invention
The present invention relates to a color separating optical system and an image pickup apparatus including the same, and more particularly, to a color separating prism for separating light from an objective lens into multiple beams, and an image pickup apparatus using the color separating prism.
2. Description of the Related Art
Broadcasting cameras and some consumer high-grade imaging cameras often use a so-called Philips type three-color separating optical system, which has an air gap between a first prism and a second prism. The Philips type three-color separating optical system is briefly described below.
FIG. 11 illustrates a conceptual diagram of the Philips type color separating optical system. In an image pickup apparatus using this type of color separating optical system, a solid-state image pickup element as represented by a CCD is used. The surface of the CCD is subjected to metal coating so as to have a relatively high reflectance. Reflection involving strong diffraction therefore occurs on the CCD surface in the case of photographing an intense light source. Part of light reflected by the CCD surface re-enters a solid-state image pickup element 1011R via an optical path (1021) of the second prism as illustrated in FIG. 12A, and appears as a ghost. FIG. 12B is a diagram expanding the ghost path (1021) of FIG. 12A along the second prism. In particular, FIG. 12B illustrates a beam that enters a reflection surface 1007 perpendicularly to be reflected. As understood from FIG. 12B, of the light reflected by the CCD, light immediately after having entered the second prism enters a surface 1006 at an incident angle β and light after being reflected by a surface 1007 enters the surface 1006 again at an angle γ, both of which are equal to an apex angle θ2 of the prism. In the case where the apex angle θ2 satisfies θ2<sin−1 (1/Ng) where Ng represents a refractive index of a glass material of the prism, light in this ghost path is not totally reflected by the surface 1006, but interference occurs between light beams reflected by the surfaces 1003 and 1006 sandwiching an air gap 1005. In the case where the apex angle θ2 satisfies θ2≧sin−1 (1/Ng), on the other hand, light in this ghost path is totally reflected by the surface 1006. No interference therefore occurs so that light is totally reflected reliably to return to the solid-state image pickup element.
FIG. 13 is a diagram illustrating a ghost path (1022) in which the incident angle to the surface 1007 slightly deviates from the right angle. In this case, the incident angle to the surface 1006 is changed, and hence ghost reflection intensity changes depending on locations. An incident angle at a point P3 is larger than an incident angle at a point P4, and hence it is understood that total reflection is more likely to occur at the point P3 while total reflection is less likely to occur at the point P4. Therefore, interference occurs at the point P4 when the total reflection does not occur. FIG. 14 illustrates a schematic diagram of a ghost appearing on an image. In a total reflection ghost region of FIG. 14, total reflection occurs at both the points P3 and P4, whereas in a region in which an interference ghost appears, interference occurs at the point P3 or the point P4. The change in θ2 changes the total reflection ghost region appearing on the screen and the region in which an interference pattern appears.
For example, Japanese Patent Application Laid-Open No. 2000-266915 discloses a beam splitting prism in which the apex angle θ2 of the second prism is appropriately set so that a ghost may be totally reflected by the incident surface 1006 of the second prism to prevent the occurrence of an interference pattern caused by the air gap.
In the conventional technology disclosed in Japanese Patent Application Laid-Open No. 2000-266915, however, although the interference pattern can be made less conspicuous, the region whose reflectance for a ghost is 100% is increased. Therefore, an image in the solid-state image pickup element for red becomes an image having a flare on the entire screen.